When ntpd is configured with rate limiting for all associations (restrict default limited in ntp.conf), the limits are applied also to responses received from its configured sources. An attacker who knows the sources (e.g., from an IPv4 refid in server response) and knows the system is (mis)configured in this way can periodically send packets with spoofed source address to keep the rate limiting activated and prevent ntpd from accepting valid responses from its sources.
When ntpd is configured with rate limiting for all associations (restrict default limited in ntp.conf), the limits are applied also to responses received from its configured sources. An attacker who knows the sources (e.g., from an IPv4 refid in server response) and knows the system is (mis)configured in this way can periodically send packets with spoofed source address to keep the rate limiting activated and prevent ntpd from accepting valid responses from its sources.
http://support.ntp.org/bin/view/Main/NtpBug3071
Mitigation: If you choose to use restrict default limited ..., be sure to use restrict source ... (without limited) to avoid this attack.
Vulmon